Passive keyless entry is a generic term for an automotive technology that allows a vehicle driver to lock and unlock a vehicle without using the corresponding SmartKey buttons. Once a driver enters a vehicle with an equipped Keyless Go SmartKey or Keyless Go wallet size card, they have the ability to start and stop the engine, without inserting the SmartKey (i.e. ignition key). A transponder built within the SmartKey allows the vehicle to identify a driver. In some cases, an additional safety feature is integrated into the vehicle, making it impossible to lock a SmartKey that has a Keyless Go feature inside a vehicle. After a few years on the market, this technology is being used ever more extensively, evolving down from luxury car manufacturers to some economy car brands.
FIG. 1 illustrates a simplified example block diagram of a known base station 100 for a passive keyless entry (PKE) system. The base station incorporates circuitry in both a digital domain 110 and an analog domain 160. The base station includes a control input signal 112 that is input to a signal (sine wave) generator 114. The signal (sine wave) generator 114 produces the basic shape of the carrier signal that is directly input into the multiplier 140. The generated carrier signal is a 125 KHz digital output signal 122. The multiplier 140 facilitates setting the desired output (power) level as well as also allowing amplitude modulation of the carrier to be applied. The multiplied (modulated) 125 KHz digital output signal 122 is then converted to an analog signal in digital to analog converter (DAC) 162. The 125 KHz analog output signal is amplified in a non-ideal amplifier 164 to produce an output signal 170.
In a PKE system, there exists a spectral noise requirement for the PKE antenna coil driver application to keep harmonics of the 125 kHz driver signal 80 dB below the main carrier signal. This requirement is particularly relevant to those harmonics that fall into the AM radio band (530 KHz-1710 KHz). With the currently available CMOS technologies, and with the currently available circuit techniques, it is not possible to build an amplifier that exhibits such extreme low-level distortion/harmonics in the output signal 170. Thus, in known base stations for PKE systems, the output signal 170 is a sine wave with relatively high, undesirable levels of harmonic content.
Thus, a need exists for a wireless transmitter, for example as used in a passive keyless entry (PKE) base station, a circuit and a method to reduce harmonic content in order to meet a spectral noise requirement.